Nickel ferrite nanoparticles-hydrogen peroxide: A green catalyst-oxidant combination in chemoselective oxidation of thiols to disulfides and sulfides to sulfoxides

Article abstract of DOI:10.1039/c4ra04095c

Nickel ferrite nanoparticles-hydrogen peroxide has been demonstrated for the first time as a green and efficient catalyst-oxidant combination in the chemoselective oxidation of thiols to disulfides and sulfides to sulfoxides. This magnetically separable catalyst was found to be reusable for five consecutive runs without appreciable change in the activity, as well as composition of the catalyst. The mechanism for the oxidation of thiols and sulfides has also been proposed. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra04095c

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Nickel ferrite nanoparticles–hydrogen peroxide: a
green catalyst-oxidant combination in
chemoselective oxidation of thiols to disulfides and
sulfides to sulfoxides†
Cite this: RSC Adv., 2014, 4, 36702
a
a
Aparna M. Kulkarni, Uday V. Desai, Kapil S. Pandit,a Makarand A. Kulkarnia
*
and Prakash P. Wadgaonkarb
Nickel ferrite nanoparticles–hydrogen peroxide has been demonstrated for the first time as a green and
efficient catalyst-oxidant combination in the chemoselective oxidation of thiols to disulfides and sulfides
to sulfoxides. This magnetically separable catalyst was found to be reusable for five consecutive runs
without appreciable change in the activity, as well as composition of the catalyst. The mechanism for the
oxidation of thiols and sulfides has also been proposed.
Received 4th May 2014
Accepted 23rd July 2014
DOI: 10.1039/c4ra04095c
www.rsc.org/advances
sulfoxides, the development of an eco-benign protocol for their
synthesis is highly desirable.
1. Introduction
In recent years, magnetic nanoparticles (MNPs), owing to
their easy separation by application of an external magnetic
eld, have emerged as a useful class of heterogeneous cata-
lysts.24,25 In particular, iron oxide-based nanoparticles such as
magnetite (Fe3O4) and maghemite (g-Fe2O3) have been previ-
ously explored as catalysts, as well as catalyst support in a few
organic transformations.26,27 Among other oxides, spinel ferrites
(MFe2O4; M ¼ Ni, Zn, Mn or Co) have received a great deal of
attention due to their applications as catalyst support, in
magnetic drug delivery, magnetic high-density storage, etc.28–31
Among spinel ferrites, nickel ferrite (NiFe2O4) with an inverse
spinel structure is an important so magnetic material with
remarkable thermal stability, large magnetic anisotropy and
moderate saturation magnetization. A literature survey revealed
that very little attention has been paid toward its catalytic
potential in organic transformations. With these observations
in mind and our continued interest in the development of eco-
benign synthetic methodologies,32 we planned to explore the
catalytic potential of nickel ferrite-nanoparticles in a few
oxidative transformations, beginning with the studies on the
oxidation of thiols, as well as suldes using hydrogen peroxide
as a green and commercially available oxidant (Scheme 1).
Our plan on the use of spinel ferrites in the oxidation of thiols
as well as suldes is based upon the concept that the catalytic
properties of AB2O4-type spinel ferrites strongly depend on the
nature of A and B ions, their charge, as well as their distribution
among the octahedral Oh and tetrahedral Td sites.33 Reithwisch
and Dumesic, during their studies on a number of spinel
structures have shown that in inverse and mixed spinel struc-
tures, there exists a rapid electron exchange between M2+ and
M3+ ions.34 Simultaneously, it is also well known that Fe2+ ions
A method for developing operationally simple and environ-
mentally benign protocols for the synthesis of intermediates
useful in chemistry, as well as in biology is a fascinating area for
research. Particularly, disuldes and sulfoxides, as well as
sulfones are a few such intermediates of common interest. For
instance, disuldes are known to play a crucial role in many
biological, as well as chemical processes, while a few disuldes
nd application in oil-sweetening processes, vulcanizing
agents, etc.1–5 Oxidative coupling of thiols is the most exploited
pathway in disulde synthesis, and varieties of reagents have
been reported for this important transformation.6–13 Similar to
disuldes, sulfoxides are also highly sought-aer owing to their
importance as valuable synthons in C–C bond forming reac-
tions, Diels–Alder reactions, as chiral auxiliaries and for their
applications in medicinal chemistry.14–16
A considerable
number of methods have been previously reported for the
controlled oxidation of suldes to sulfoxides using H2O2 in
combination with human hemoglobin, TMSCl, ZnBr2, ZrCl4,
etc.17–20 Quite recently, hydrogen peroxide in combination with
WO3 nanoparticles supported on MCM-48, TiO2 nanoparticles
and magnetic nanoparticle-immobilized N-propylsulfamic acid
have also been reported for the oxidation of suldes to sulfox-
ides.21–23 Although most of these methods are efficient, in view
of chemical as well as biological signicance of disuldes and
aDepartment of Chemistry, Shivaji University, Kolhapur-416004, India. E-mail:
uvdchem2011@gmail.com; Fax: +91-0231-2609333
bPolymer Science and Engineering Division, CSIR, National Chemical Laboratory,
Pune-411008, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra04095c
36702 | RSC Adv., 2014, 4, 36702–36707
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Scheme 1 Nickel ferrite catalyzed oxidation of (a) thiols to disulfides
and (b) sulfides to sulfoxides.
Fig. 1 XRD pattern of NiFe2O4–MNPs: (a) before use; (b) after use; (c)
simulated XRD.
The absence of extra lines in the present pattern conrms the
single-phase formation of nickel ferrite. To corroborate the
formation of NiFe2O4 as single phase, using the structural data
reported by Kremenovic et al. for the cubic phase,37d theoretical
XRD patterns of NiFe2O4 were simulated (Fig. 1c) and were
compared with the experimentally observed XRD pattern of the
NiFe2O4 nanoparticles prepared in this study. It reveals that the
simulated XRD pattern of the NiFe2O4 is in good agreement
with the powder XRD patterns of the NiFe2O4 collected before
and aer the catalytic applications (Fig. 1a and b). The lattice
parameter calculated using the XRD data was found to be 8.325
Scheme 2 Plausible mechanism for oxidation of (A) thiols and (B)
sulfides.
during oxidation with H2O2 to Fe3+ ions, generate HO radical
and OHꢀ ion.35,36 Based on these facts, we summarised that the
nickel ferrite nanoparticles–hydrogen peroxide combination
would serve as a useful catalyst-oxidant combination in the
oxidation of thiols, as well as suldes. It was postulated that
nickel ferrite on reaction with hydrogen peroxide would generate
OH radical, which would subsequently react with thiol to furnish
thiol radicals, and dimerization of thiol radicals would furnish
disulde (Scheme 2A). In addition, if this concept proves true,
the planned protocol can be safely extended towards oxidation of
suldes to sulfoxides (Scheme 2B).
˚
A and was in good agreement with the value reported for the
uncoated ferrite.37b,c The crystallite size of the sample was
calculated from the corrected FWHM value of (311) reection
using Scherrer's equation and was found to be ꢂ20 nm. The
SEM image of the catalyst (Fig. 2) indicates the presence of
nickel ferrite nanoparticles in aggregated form. Thus, no
assertion could be made about the exact morphology of the
particles. Hence, the catalyst was further analysed using TEM
analysis. The TEM image (Fig. 3) shows cubic morphology of the
nanoparticles of size ranging between 14 and 20 nm. The
particle size estimated from the TEM image is in good agree-
ment with that predicted using XRD data. Fig. 4 shows the
hysteresis loop of the sample. A very narrow hysteresis cycle
with a saturation magnetization value of 41.10 emu gꢀ1 indi-
cates the ferrimagnetic behavior of the catalyst, which allows
2. Results and discussion
Initially, nickel ferrite (NiFe2O4) nanoparticles were synthesized
employing controlled co-precipitation method. In a typical
procedure, an aqueous 4 M solution of sodium hydroxide was
added dropwise to a well-stirred solution of the stoichiometric
amount of NiCl2$6H2O and FeCl3$7H2O in water. The resultant
precipitate was ltered and washed several times with de-
ꢁ
ionized water. It was dried under vacuum at 60 C for 6 h and
resultant nickel ferrite particles were sintered between 800–
1000 ꢁC for 2 h. The prepared catalyst was characterized by
various spectral methods.
The powder XRD pattern for nickel ferrite is shown in Fig. 1a.
The peaks in the XRD pattern were indexed in light of the
natural spinel structure of MgAl2O4. According to the spinel
structure, the planes that diffract X-rays are (220), (311), (400),
(422), (511) and (440). All of the detectable peaks are indexed as
NiFe2O4 with an inverse spinel structure and are in good
agreement with the standard data (44-1485 IC DD; JCPDS).37a Fig. 2 SEM images of NiFe2O4–MNPs: (a) before use; (b) after use.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 36702–36707 | 36703

View Article Online
RSC Advances
Paper
oxidation of thiols. To begin with, 4-bromothiophenol 1a was
selected as a model substrate, nickel ferrite nanoparticles as a
catalyst and H2O2 as an oxidant (Scheme 1a). A set of reactions
were carried out employing different catalysts, as well as an
oxidant concentration in different solvents. The results
summarized in Table 1 reveal that complete oxidation of 1a to
corresponding disulde 2a takes place in extremely short time
with the use of 15 mol% of the catalyst, 2.5 equivalent of 30%
H2O2 and acetonitrile as the solvent (entry 8, Table 1). On the
other hand, in absence of the catalyst or oxidant, the reaction
does not proceed to completion (entry 15 &16 Table 1). Under
the established reaction conditions, the oxidation of 1a was also
carried out using simple ferrite (Fe3O4), as well as copper ferrite
as catalysts (entries 13 and 14, Table 1). Both these catalysts
were found to be effective; however, the oxidation was quite
slow and considerably less amount of disulde 2a was obtained.
Fig. 3 TEM image for NiFe2O4 nanoparticles.
Table 2 Nickel ferrite catalyzed oxidation of thiols to disulfidesa
Entry Product
Time (min) Yieldb (%)
a
5
5
96
94
b
Fig. 4 Hysteresis loop for NiFe2O4–MNPs.
c
5
5
94
92
easy separation of the catalyst using an external magnet for
possible reuse.
d
Aer adequate characterization of the catalyst, we initially
planned to explore the catalytic potential of nickel ferrite in the
e
5
93
Table 1 Optimization of reaction conditions for oxidative coupling of
4-bromothiophenol, 1a
No. Catalyst (mol%) H2O2 (equiv.) Solvent Time (min) Yielda (%)
f
5
7
88
89
1
2
3
4
5
6
7
8
9
25
20
15
10
5
15
15
15
15
5
5
5
5
5
4
3
2.5
2
1.5
2.5
2.5
2.5
2.5
2.5
—
CH3CN
CH3CN
CH3CN
CH3CN 5, 15
CH3CN 20
CH3CN
CH3CN
CH3CN
CH3CN 15
CH3CN 30
THF
CH2Cl2
CH3CN 40
CH3CN 40
CH3CN 120
CH3CN 120
5
5
5
98
98
96
90, 92
80
96
96
96
85
60
50
90
80
85
20
30
g
5
5
5
h
i
8
95
94
10
10 15
11 15
12 15
13 20b
14 20c
30
5
j
10
92
15
16 15
—
a
Reaction conditions: thiol (2 mmol), catalyst (15 mol%, 0.07 g),
b
a
acetonitrile (5 mL), H2O2 (2.5 equiv., 30%, 0.6 mL), RT. Yields refer
to pure isolated products.
Reaction conditions: 4-bromothiophenol (1 equiv.), catalyst (NiFe2O4),
solvent, H2O2, RT. b Using Fe3O4 as catalyst. c Using CuFe2O4 as catalyst.
36704 | RSC Adv., 2014, 4, 36702–36707
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Table 3 Chemoselective oxidation of sulfides to sulfoxidesa
Table 3 (Contd.)
Yieldb (%)
Entry
Product
Time (h)
Yieldb (%)
Entry
Product
Time (h)
2.0
24
6.0
2.0
90
90
NRc
92d
a
n
2.0
2.5
92
b
2.0
87
o
88
c
2.5
3.5
1.5
90
88
90
a
Reaction conditions: sulde (2 mmol), catalyst (15 mol%, 0.07 g),
b
acetonitrile (5 mL), H2O2 (2.5 equiv., 30%, 0.6 mL), RT. Yields refer
to pure isolated products. In absence of oxidant. Using 5.0 equiv.
(1.2 mL) 30% H2O2.
c
d
d
e
Based upon this experimental data, nickel ferrite nano-
particles were selected as the catalyst to examine the generality
of the reaction conditions, as well as scope of the protocol.
Accordingly, a few representative aromatic, as well as aliphatic
thiols were subjected to oxidation under the optimized reaction
conditions. In each case, corresponding disulde, 2b–j, were
obtained in excellent yield, with high purity in a very short time
(Table 2).
f
3.5
85
Encouraged with this initial success on the oxidation of
thiols to disuldes, we then turned our attention towards the
oxidation of suldes to sulfoxides (Scheme 2B). Accordingly,
thioanisole, 3a, was chosen as the substrate, and it was
observed that under the reaction conditions established for the
oxidation of thiols, the oxidation of thioanisole proceeds
smoothly to furnish corresponding sulfoxide as the only
product (TLC), but it requires slightly longer time (2 h). Most
interestingly, either upon the use of an excess amount of
oxidant or upon stirring the reaction mixture overnight, we did
not notice the formation of sulfone as an over-oxidation product
(TLC, Table 3, entry a). Hence, without the modication of the
reaction conditions, we planned to examine the scope of the
developed protocol. Initially, thioethers bearing electron-
donating as well as electron-withdrawing groups on aromatic
rings were subjected to oxidation under the established reaction
conditions. In each case, corresponding sulfoxide was obtained
in excellent yield (Table 3, entries 4a–f). Thus, to check the
functional group compatibility in this oxidative transformation,
thioethers containing alcohol, nitrile, aldehyde, ketone, and
ester groups, as well as an unsaturation centre were then sub-
jected to oxidation. In all cases, corresponding sulfoxides were
obtained in excellent yield (4g–o, Table 3), and no side products
or over-oxidation products were observed.
g
2.0
2.5
84
85
h
i
j
3.0
3.0
90
87
k
l
2.0
2.5
3.5
85
87
90
m
Heterogeneously catalyzed reactions generally offer an
advantage of the possibility of catalyst reuse. Hence, upon
completion of the model reaction, the catalyst was separated by
the application of an external magnet (Fig. 5). The recovered
ꢁ
catalyst was washed with acetone, dried in oven at 80 C for 3
hours and reused for the next run. It was observed that the
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 36702–36707 | 36705

View Article Online
RSC Advances
Paper
catalyst could be recycled efficiently for ve cycles without behavior of the catalyst was tested. Thus, upon completion of
appreciable loss in its activity (Fig. 6). So as to check the stability the model reaction, the catalyst was separated and the resultant
of the catalyst during recycling, the SEM and XRD spectra of the organo-aqueous phase was examined for the presence of Fe as
catalyst recovered aer h cycle were recorded (Fig. 1b and well as Ni by performing spot tests, as well as through AAS
2b). Comparison of the SEM images and XRD pattern reveals analysis. In both the cases, the results obtained were negative.
that although the morphology of the catalyst does not change Hence, the composition of the catalyst before and aer use was
appreciably (Fig. 2a and b), as evidenced from the increased then conrmed from the EDAX spectra (Fig. 7). To our delight,
peak width in the XRD pattern (Fig. 1a and b), the grain size of we did not observe any appreciable change in the composition
the particles obtained aer use decreased.38 Lately, the leaching of the catalyst.
3. Conclusion
In conclusion, we have demonstrated for the rst time, the
use of nickel ferrite nanoparticles–H2O2 as a green catalyst-
oxidant combination in oxidation of thiols to disuldes
and suldes to sulfoxides at ambient temperature. This
magnetically separable catalyst can be prepared by a very
simple procedure using inexpensive precursors, and the
catalyst can be reused for ve cycles without any signicant
loss in its catalytic activity. Thus, the protocol serves as a
Fig. 5 Easy separation of the catalyst by application of external
magnet.
useful alternative in the synthesis of disuldes and
sulfoxides.
4. Experimental
All the chemicals were purchased from Aldrich or S.D. Fine
Chemicals, Mumbai. 1H and 13C NMR spectra were recorded
using a Bruker Avance-II spectrometer. X-ray powder diffraction
was performed on a PHILIPS (PW3710) X-ray diffractometer
with Cu Ka1 radiation (l ¼ 1.5424 A.U.). SEM micrographs were
taken on a JEOL-JEM – 6360 microscope. Magnetization curves
of nickel ferrite-nanoparticles were obtained using a vibrating
sample magnetometer (VSM), while EDAX spectra were recor-
ded using a Genesis XM – 2i EDX system. TEM studies
were performed using Philips CM 200 with operating voltage:
Fig. 6 Recyclability of the catalyst for oxidation of sulfides.
20–200 kv.
Fig. 7 EDAX images of NiFe2O4–NPs: (a) before use; (b) after use.
36706 | RSC Adv., 2014, 4, 36702–36707
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
19 X.-F. Wu, Tetrahedron Lett., 2012, 53, 4328–4331.
20 K. Bahrami, Tetrahedron Lett., 2006, 47, 2009–
2013.
21 D. H. Koo, M. Kim and S. Chang, Org. Lett., 2005, 7, 5015–
5018.
22 R. Mohammad, G. h. Rajabzadeh, G. Rajabzadeh,
S. M. Khatami, H. Eshghi and A. Shiri, J. Mol. Catal. A:
Chem., 2010, 323, 59–64.
23 A. Rostamia, B. Tahmasbia, F. Abedib and Z. Shokri, J. Mol.
Catal. A: Chem., 2013, 378, 200–205.
24 S. Laurent, D. Forge, M. Port, A. Roch, C. Robic and L. V. Elst,
Chem. Rev., 2008, 108, 2064–2110.
25 D. Lee, J. Lee, H. Lee, S. Jin, T. Hyeon and B. M. Kim, Adv.
Synth. Catal., 2006, 348, 41–47.
26 (a) H. Yoon, S. Ko and J. Jang, Chem. Commun., 2007, 14,
1468–1470; (b) N. V. Long, Y. Yang, M. Yuasa, C. M. Thi,
Y. Cao, T. Nann and M. Nogami, RSC Adv., 2014, 4, 6383–
6390.
27 J. Safari, Z. Zarnegar and M. Heydarian, Bull. Chem. Soc. Jpn.,
2012, 85, 1332–1338.
28 B. Karimi and E. Farhangi, Chem.–Eur. J., 2011, 17, 6056–
6060.
29 K. K. Senapati, S. Roy, C. Borgohain and P. Phukan, J. Mol.
Catal. A: Chem., 2012, 352, 128–134.
30 M. P. Pileni, Adv. Funct. Mater., 2001, 5, 323–336.
31 Q. Song and Z. J. Zang, J. Am. Chem. Soc., 2004, 126, 6164–
6168.
32 M. A. Kulkarni, K. S. Pandit, U. P. Lad, U. V. Desai,
P. P. Wadgaonkar and Z. J. Zang, C. R. Chimie, 2013, 16,
869–875.
33 Structural Inorganic Chemistry, ed. A. F. Wells, Clarendon
Press, Oxford, 1984.
34 D. G. Reithwisch and J. A. Dumesic, Appl. Catal., 1986, 21,
97–109.
35 Y. Ogata and Y. Sawaki, in Organic Synthesis by oxidation with
metal compounds, ed. W. J. Mijs, C. R. H. I. Jonge de, Plenum
Press, New York, 1996, p. 480.
General procedure for the oxidation of thiols to disuldes and
suldes to sulfoxides
To a well-stirred solution of thiol, 1 (or sulde, 3) (2 mmol) and
H2O2 (30%, 2.5 equiv., 0.6 mL) in acetonitrile (5 mL), nickel
ferrite-nanoparticles was added (15 mol%, 0.07 g), and stirring
was continued. Upon completion of oxidation (TLC), the cata-
lyst was separated using an external magnet. The reaction
mixture, was ltered under vacuum, and the resultant residue
was chromatographed over a short column of silica gel. Elution
with hexane–ethyl acetate (95 : 5%, v/v) furnished pure disul-
de, 2 (or sulfoxide, 4) in excellent yields.
All the synthesized compounds are known. However, they
were characterised by NMR spectroscopy. For original NMR
spectra of sulfoxides, please see the ESI.†
References
1 S. Oae, Organic Sulfur Chemistry; Structure and mechanism,
CRC, Boca Raton, FL, 1991.
2 R. J. Cremlyn, An Introduction to Organosulfur Chemistry,
Wiley & Sons, New York, 1996.
3 S. Basu, S. Satapathy, M. P. Singh and A. K. Bhatnagar, Catal.
Rev.: Sci. Eng., 1993, 35, 571–576.
4 S. M. S. Chauhan, A. Kumar and K. A. Srinivas, Chem.
Commun., 2003, 18, 2348–2349.
5 K. Ramdas and N. Srinivasan, Synth. Commun., 1995, 25,
227–234.
6 J. Drabowicz and M. Mikolajezyk, Synthesis, 1980, 1, 32–34.
7 W. A. Pryor, D. F. Church, C. K. Govindan and G. Crank, J.
Org. Chem., 1982, 47, 156–159.
8 A. McKillop and D. Koyunu, Tetrahedron Lett., 1990, 31,
5007–5011.
9 J. L. G. Ruano, A. Parra and J. Alleman, Green Chem., 2008,
10, 706–711.
10 M. Carril, R. SanMartin, E. Dominguez and I. Tellitu, Green
Chem., 2007, 9, 315–317.
11 A. Dakshinamoorthy, M. Alvaro and H. Gracia, Chem.
Commun., 2010, 46, 6476–6478.
12 A. Saxena kumar and A. S. Mozumdar, J. Mol. Catal. A: Chem.,
2007, 269, 35–40.
36 A. M. Dumitrescu, P. M. Samoila, V. Nica, F. Doroei,
A. R. Iordan and M. N. Palamaru, Powder Technol., 2013,
243, 9–17.
37 (a) R. Sailer, G. McCarthy, ICDD Grant in Aid, 1992; (b)
W. Wheicheng, L. Shuo and W. Yiyun, J. Mater. Sci.
Technol., 2008, 24, 1761–1765; (c) N. Iimie, E. Rezlescu,
P. D. Popa and N. Rezlescu, J. Optoelectron. Adv. Mater.,
2006, 8, 1016–1018; (d) A. Kremenovic, B. Antic,
M. Vucinic-Vasic, P. Colomban, N. Bibic, V. Kahlenberg
and M. Leoni, J. Appl. Crystallogr., 2010, 43, 699–
709.
13 N. Iranpoor, H. Firouzabadi and A. Pouradi, Tetrahedron,
2002, 58, 5179–5184.
14 M. C. Carreno, Chem. Rev., 1995, 95, 1717–1760.
15 N. Khiar, I. Fernandez and F. Alcudia, Tetrahedron Lett.,
1993, 34, 123–126.
16 I. Fernandez and N. Khiar, Chem. Rev., 2003, 103, 3651–3705.
17 A. Kumar and Akanksha, Tetrahedron Lett., 2007, 48, 7857–
7860.
18 K. Bahrami, M. M. Khodaei, B. H. Youse and M. S. Arabi,
Tetrahedron Lett., 2010, 51, 6939–6941.
38 N. Mo, Y. Y. Song and C. E. Patton, J. Appl. Phys., 2005, 97,
93901.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 36702–36707 | 36707